Traveling wave tube interaction circuit



Aug. 9, 1960 A. AsHKlN TRAVELING WAVE TUBE INTERACTION CIRCUIT 4Sheets-.Sheet 1 Filed NOV. 21, 1956 QWRDO IIIII .l E QN N @Dx /NVEN Tof?A. ASHK/N Aug. 9, 1960 A. AsHKlN TRAVELING WAVE TUBE INTERACTION CIRCUITFiled NOV. 21, 1956 4 Sheets-Sheet 2 /Nl/E/v ro@ A. ASHK/N AHORA/5v Aug.9, 1960 A. Asl-:KIN

TRAVELING WAVE TUBE INTERACTION CIRCUIT 4 Sheets-Sheet 3 Filed Nov.21.11956 kbos" /NVE/vroR A. ASHK/N A ATTORNEY u8- 9, 1960 I A. AsHKiN2,948,828

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r Y i f n I d AML-fg 7 ATTORNEY rents. ;a traveling wave tube is quitewell understood and is dis- .cussed in an article by H. Hetfner,Analysis of the Backward Wave Traveling Wave Tube, Proceedings of thej'I.R.E., volume 42 (June 1954), pages 930 through 937. `Theseoscillations can occur in a helix type of tube, be- -cause the helix,where uniformly wound, is actually a periodic structure and with a helixof uniform pitch, the

TRAVELI'NG WvErUE nvTERAcTIoN yCIRCUIT Arthur Ashkin, Far Hills, NJ.,assi'gnor to Ben 'reiephone Laboratories, Incorporated, New York, NX., acorporation of New York ined Nov. 2'1, 1956, ser. No. 623,623

. 16 claims. (ci. sis- 3.6)

This invention relates to traveling wave tubes, which may be describedas electron tubes utilizing a wave transmission circuit for propagatingelectromagnetic waves in eld coupling relation with an electron beam topermit interchange of energy between the transmission circuit and theelectnon beam along several wavelengths of the circuit, It relatesparticularly to such tubes which employ in the transmission circuit ahelical wave path to reduce the electromagnetic wave velocity -along thepath of the electron beam so that it is substantially synchronous withthe beam.

One of the diiiiculties encountered in attempting to use such tubes ashigh power amplifiers is the production of .unwanted backward waveoscillations at high beam cur- The production of backward Waveoscillations in `spaces between adjacent turns can be regarded as inter-;acting gaps periodically located along the path of the .electron beamwhich permit spatial harmonic interaction ,between the beam and thehelix circuit. Due to the fact that the impedance to the beam is lowerfor the sapltal ,harmonic mode of operation than for the fundamentalmode, harmonic operation does not occur in helix type However, when, for

v.wave oscillation.

It is the object of this invention to provide means in V.the helix typeof traveling wave tube for preventing inter- .action which will causethe objectionable operation in the spatial harmonic mode withoutimpairing operation in :the desired fundamental mode.

This is accomplished, according to the invention, by utilizing in thewave transmission circuit a helix wound lwith a continuously varyingpitch in the region of interaction with the electron beam to destroy theperiodicity of the interacting gaps between adjacent turns and sodislcourage operation in the harmonic mode with consequent building upof backward wave oscillations. In order to retain eiiicient operation ofthe tube in the fundamental mode, the effect of the variation in thepitch of the helix upon the wave velocity therealong is counteracted bycontinuously varying the loading of the helix along its length torestore the desired normally constant phase velocity characteristicalong the interaction region. Any

means suitable for appropriately varying the loading of` the helix maybe employed. At present, it appears preferable to use either dielectricmaterial properly distributed along and in the eld of the helix, or asecond Patented Aug. 9, 1960 ic i 2 helix extending along and variablycoupled to the first helix having the variable pitch.

The invention and its application in specific embodiments will be betterunderstood from the following description and the accompanying drawingsof which:

Fig. 1 is aschematic drawing illustrating an embodiment of the inventionin a traveling wave tube utilizing in the wave transmission circuit ahelix of uniform diameter 'and varying pitch variably loaded along itslength by means of dielectric material to maintain uniform phasevelocity along the circuit;

Fig. 2 is a schematic drawing illustrating an embodiment of theinvention ina traveling wave tube utilizing in the wave transmissioncircuit a rst (inner) `helix of uniform diameter and varying pitch whichis variably loaded valong its length by means of a second (outer) helixwhich is coaxial with the first, has a varying pitch and is of varyingdiameter for variable coupling to the iirst helix to maintain uniformphase velocity along the circuit in the principal region of interactionwith the electron beam;

Fig. 2A is a diagram indicating relative phase velocities at pointsalong the lengths'of the helices of Fig. 2 in the principal interactionregion when the helices are separated from each other (uncoupled) andalso under the two possible modes of operation when coupled as in Fig.2;

Fig. 2B is a diagram indicating relative phase velocities along thelengths of the helices of Fig. 2 in'both the coupling and the principalinteraction regions when the helices are separated from each other(uncoupled);

Fig. 3 illustrates schematically an embodiment of the invention in atraveling wave tube utilizing a tubular electron beam and in the wavetransmission circuit a first (inner) helix of uniform diameter andvarying pitch variably loaded along its length by means of. a second(outer) helix coaxial with'the first and of uniform diameter and varyingpitch for variable coupling to the iirst helix to maintain uniform phasevelocity along the cir- Acuit in the principal region of interactionWith the elec# Fig. 3 to maintain uniformphase velocity, However,

whereas in Fig. 3 the pitches of the inner and outer helices increaseand decrease respectively with the length along the entire length of theprincipal interaction region, `and the uncoupled phase velocities aresimilar in the center,

in Fig. 4 the pitches of the inner and outer helices in-f crease anddecrease, respectively, with the length from` one end to the center ofthe principal interaction region, L and then decrease and increase,respectively, with the.v length from the center of the principalinteraction region, to the yother end, and the uncoupled phasevelocities are,

similar at the two ends;l

Figs. 4A and 4B are diagrams similar to FigsrSA andi, 3B but indicatingrelative phase velocities at points along;

the lengths of the helices of Fig. 4; and Y Figs. 5 and 5A illustrateVin perspective and longit dinal section, respectively, an embodiment ofthe inven tion in a magnetron type of traveling wave tube. Herel isshown a single iiattened variablepitch helix variably loaded by means ofa tapered dielectric block to maintain constant phase velocity along thelength ofthe helix,fas

is shown in Fig. l2

To simplify thedrawings, the usual vacuum tight'enve' lope required,potential sources, the commonly employed electron beam focusing meansextending along the beam path (except in Fig. and energy absorbing meansassociated with the circuit in the interaction region to preventoscillations due to reected energy are not shown.

The various-figures, illustrative of -the several embodiments of theinvention described, are intended-merely to clarify the descriptionsland consequently-'do not Vindicate-sizes ortpropo'rtions. 'Forinstance, the tubes would ordinarily be much more elongated thanshown,rtherle'ngth indicatedas interaction region would be much longerin proportion tothe lengths'of the couplers-than is shown, and't'hediagramsshowing r'elationsbet'weenphase velocity and length along the-helices indicate only-thecha'racter ofthe variations,`not actualdimensions or slopes. Furthermore, the portion of the length of eachItube marked rinteraction lregion-'fis theprin'cipal interaction regionwhere thewavephase velocity is kept constant and vvirtually all `of theamplilication takes place. The relatively `small interaction in thecoupler regions is neglected.

-Referringnowfto'Fig 1, internal elements of a beamhelix type oftraveling wave tube, according to one ernbodiment of the invention,areillustrated schematically in the formfof a longitudinal section. VFromthe electrongun comprising cathode 1, heater 2, focusing andaccelerating electrodes 3 and 4, the electron Vbeam 5 is projected alongthe axis of and in coupling relation with the helix wave transmissioncircuit 6 to the electron collector 7. The rinput'andoutput connectionsto the helix are indicated at 8` and 9, respectively. In accordance withthe invention, the helix 6 is wound with continuously varying pitch todestroy the periodicity of the interacti'nggapsbetweenv turns, randthereby discourage the building up of the undesirable backward waveoscillations. Further in accordance with the invention, a block ofdielectric material 10 tapered gradually but not necessarily uniformwith length is disposed along and coupled to the helix toprovi'deloading of the helix in varying amount along its length to compensatefor the effect upon phase velocity of the varying pitch of the winding.This loading is so varied that the phase velocity of wave propagation`along'the Vthe length of the helix is substantially uniform inspite-,of the varying winding pitch. Thus, by varying thelhelix windingpitch, the unwanted backward wave oscillations are avoided and, 'bycompensating for such variations to maintain constant wave phasevelocity, the'normal'functionin'g of the traveling wave tube isretained. a

It may be noted that while the desired variation in loading by nthedielectric 4material 10 is shown as 'being accomplishedjby taperingthedblock of dielectric material, itmay be'had also by other suitablemeansysuch as by positioning of the block or by varying the compositionof the material. Also, the direction of the tapering -of the block ofdielectric 10- and of 'the variation in pitchuof the helix 6 may be thereverse of what is shown in the figure.

Fig. 2*'is an illustration showing in the same general manner -as Fig. lanother embodimentof the invention ina beam-helix type of traveling wavetube. Where appropriate, designations the same as in Fig. l areemployed. In this embodiment the varying loading of the inner helix 21,having gradually decreasing pitch along the Iength'of the tube indicatedas interaction region, is had by means of an outer helix 22., preferablywound oppositely to the winding of helix 21 to enhance the mutualcoupling. -Terminations not'shown may be attached to the otherwise freeends of the inner helix 21. Helix 22 also has agradually decreasingpitch along the interaction region, but has additionally a graduallychangingu-diameter to provide varying coupling between the twohelices inthat region. This coupling is adjusted to obtain a constant phasevelocityalong the interaction region.

The gradually decreasing pitch of the inner helix 21 corresponds toV agradually decreasing phase velocity in the uncoupled state, that is,separated from the inuence of helix 22. The portion of the outer helix22 in the interaction region `also having gradually decreasing pitchlikewise has a decreasing phase velocity in the unconpled state or whenseparated from the influence of helix 2l. The portions of helices 21 and22 in the i.teraction region are designed to have the same phasevelocity characteristic when uncoupled or separated from each other.That is, at each :point along the length in the interaction region l,the two helices have the same phase velocity when uncoupled,a's isindicated bythe solid line graph a of Fig. 2A.

Such a system of two coupled helices 'has two modes of wave propagation,one faster than that of the individual helices and one slower. The upperand lower dotted line graphsb and c of Fig. 2Ashow phase characteristicstypical of the faster and slower modes of propagation, respectively, ofvthe system consisting of the two helices 21 and `22'coupled together"as shown in Fig. 2.

Fig. 2A thus shows three phase velocity,characteristics typical ofthehelices Vof Fig. .2, phase velocity being plotted against'length l alongthe yhelices in .the` region indicated interaction regionin Fig.\2 whereboth helices have varying pitch.

The coupling between the two. helicesmay be adjusted so that the phasevelocity of either `theifasteror the slower modeofpropagation-risfconstant along the length I. in this particularillustrative embodiment, the coupling between the two helices isadjusted so that the phase velocityof the faster'mode of thecoupledsystem is constant along thelength of'theinteraction region, asis'shown lbythegraph 'b ofFig. 2A. .In this manner, the desiredobjective'of maintaining?constantzphase velocity alongthe interactionregionwhile at the same time employing a -helical'wave circuit withnon-uniform pitch to avoid unwanted backward wave oscillation isattained. To obtain this-result in operation it is, ofcourse, `necessarythat the coupled helix system operate in thedesired mode. In thisinstance, itis.therefore'necessaryto.provide means lto launch the fastmode at the input of the helices and to extract the power in the fast`mode at the output end. This may be doneby means of `what have beentermed step couplers, which are short lengths of appropriately coupledhelices, attached to cacherd of the portions of the two kcoupled heliceslocated in the interactionregion indicated-.in Figs. 2 and 2B. lt may benoted VinI Fig. 2 that the short lengths of helcescom prising thecouplers are `actually extensionsofand partsv of the coupled helices 21and 22. Such couplers 'are described in US. Patent 2,885,593,"issued May5, 1959, of J. S. Cook.

'Cook refers to each-suchcoupler as a transducer section, and teachesthat a desired mode of propagation may be excited in a pair of coupledtransmission lines by associating therewith a properly designedtransducer' section of lines as an-extensionof the pair of couple'lines, and applying an 'input wave to the end of one of the two linesforming the transducer section. The two lines forming the transducersection must be chosen to have different average characteristic phasepropagation constants. The faster of the two normal modes of the coupledtrans mission lines may then be launched therein b v exciting, thecomponent line of the input transducer section having'the faster averagephase velocity. and conversely, the slower normallmode may be` launchedby exciting the component line of the input transducer section havingtheslower average phase velocity. Similarlypfrom reciprocityconsiderations, the faster of the two lines forming an outputtransducerlsection will be excited selectively when the fasternormalirnode is in the main coupled line, and the slowerline when theslower modeis in the main coupled line. Reference can rbe made to thisabove-cited patent fora detailed description of the principles ofoperation and design of these couplers or transducer sections.

the proper mode of wave propagation, that is, the mode for which thephase velocity has been made constant (such as indicated by the graph bof Fig. 2A). It may be seen from Fig. 2B, which shows typical variationsin phase velocity along the helices (when uncoupled) in the interactionand coupler regions, that the coupler sections of helix 22 (the outerhelix) have higher phase Velocity characteristics than the couplersections of helix 21 (the inner helix), and therefore, to operate thesystem in the faster mode (for which the phase velocity has been madeconstant), input and output connections are made to the coupler sectionsof the outer helix. If it were desired to operate the system in theslower mode of wave propagation, appropriately dilferent couplersections or connections thereto would be employed as taught by Cook inthe aferomentionad patent.

` Fig. 3 illustrates in the same general manner as Fig; 2 anotherembodiment of the invention in a traveling wave tube utilizing twocoupled helices, preferably wound oppositely, to efect'variable loadingto maintain constant wave phase velocity along the interaction region.Where appropriate, designations are the same as for Figs. 1 and 2.However, in this ligure the cathode and other elements of the electrongun are arranged to pro-` ject a tubular beam of electrons through theannular space between the two coupled helices 31 and 32 to the collector7, although `an axial beam as shown in Fig. 2 could be used. In thisembodiment is a constant diameter inner helix 31 with pitch and phasevelocity gradually increasing along its length in the uncoupled state.The outer helix 32, however, has a pitch and phase velocity decreasingwith length along the interaction region and, unlike the outer helix ofFig. 2, it has a constant diameter. Since the separation of the phasevelocities of these helices (when uncoupled) varies with length, theeffective coupling varies with length too when they are coupledtogether. This coupling is weakest at the ends of the interaction regionand strongest in the middle. The variations of the phase velocity ofeach of the helices can be adjusted to achieve a situation in which thecoupled system has a mode of wave propagation with constant phasevelocity whereby, according to the invention, efficient traveling wavetube operation may be had together with the advantage of non-uniformhelix pitch in preventing backward wave oscillation.

Fig. 3A illustrates typical phase velocity variations along the length l(the interaction region) of the inner and outer helices 31 and 32. Thesolid line graphs a1 and a2 show variations of phase velocity along thetwo helices when uncoupled, and the dotted line graphs b and c showphase velocity variations along the two helices forthe faster and slowermodes ofwave propagation when the helices are coupled together, as inFig. 3. The phase velocity may be and is shown constant for both modesof propagation.

ln order to operate the Fig. 3 embodiment in the desired mode,step-couplers or transducer sections are shown at each end of theinteraction region, as has been explained in connection with thedescription of Fig. 2.

Typical variations in phase velocity along the helices in the couplerand the interaction regions when the helices are uncoupled are shown inFig. 3B. The outer helix portions of the couplers have the higher phasevelocities,

and to these the input and output connections are made in Fig. 3 tooperate .the system in the faster velocity mode of wave propagation. Asmentioned in the description of Fig. 2, terminations not shown m-ay beconnected to the otherwise 'free ends of the inner helix 31.

Fig. 4 illustrates an embodiment of the invention which, like Fig. 3,employs two helices, each with constant diameter. Here, however, `alon-gthe lengthjoff the interaction region, the pitch and phase velocity ofthe inner helix increase and then decrease to the initial value, whilethe pitch and phase velocity of the outer helix along that regiondecreasey and then increase to the initial value. As with Fig. A3, when-coupled together, these sections of helix can have modes of propagationwith constant phase velocity. n

Where appropriate, the designations on Fig. 4 are the same as for theprevious gures, and it may be noted that, an axial electron 'beam isshown as in Fig. 2, although a tubular beam as in Fig. 3 could be used.The inner yand outer coupled helices are designated 41 and 42,respectively. As with Fig. 3, since the separation of the phasevelocities of these two helices (when uncoupled) varies with length, theeffective coupling varies with length'too when they a-re coupledtogether. Here, however, the coupling is strongest Iat the ends of theinteraction region, and weakest in the middle. Again (as with Fig. 3),the variations of the phase velocity of each of the helices can beadjusted so that the coupled system h-as a mode of wave propagation withconstant velocity whereby the objective of the invention may beachieved.

Fig. 4A illustrates typical phase velocity variations along the length l(the interaction region) of the, inner Iand outer helices 41 land 42.The solid line graphs a1 and a2 show variations of phase velocity alongthe two helices uncoupled, and the dotted line gaphs b and c show phasevelocity variations along the two helices for the faster and ,slowerVmodes of wave propagation when the helices are coupled together, as inFig. 4.

To operate the Fig. 4 embodiment in the desired mode, step-couplers,l ortransducer sections, are shown at each end of the interaction region, ashas lbeen explained in connection with the descriptions off Figs. 2 and3. Typical variations in phase velocity along the helices in the couplerand interaction regions when the helices are uncoupled are shown in Fig.4B. 'The outer helix portionsl of the couplers 'have the higher phasevelocities, and these are used for the input and output connections inFig. 4 to operate the system in the Vfaster Velocity mode of Wavepropagation. As previously mentioned, terminations not Ishown may beconnected to the otherwise free ends `of the inner helix 41. Figs. `5land 5A illustrate an embodiment of the inveny tion in a linearmagnetron type ofutravelinggwave tube employing a 'helix type of wavepropagating circuit. A

flattened helix 51 is shown as an example. In accord? lance with ltheinvention, the pitch of the helix is varied continuously along its'length, land further by way of example, a tapered block of dielectricmaterial10 is located along and in proximity to the helix to variablyload'the into Va magnetron, crossed electric `and magnetic field, typeIn Figs. 5 and 5A, the fiat electron beam 50 s projected from thecathode 1 of the electron gun to the collector 7 along the helix 51between the 'helix and the oppositely polarized plate 52. The magneticfield producin'g means, pole pieces 53 and 54, produce a magnetic fieldin the space between plate 52 and the helix which is perpendicular tothe electric field there and to lthe direction of travel of the electronbeam 50. The region of interaction between the electron beam 50 and thehelix 51 is along the space between plate 52 and the helix. The focusingstrips 55 are maintained at different direct voltages with respect tothe helix 51, and are adjusted to' counteract the Varialble magneticforce in the fringe field.

region by maintaining the proper ratio of electric to magY netic fieldin the fringe magnetic field. This means for focusing an electron beamprior to its entrance into the magnetic `feldrofthe.interaction space ina linear magnetron is described in my US. Patent 2,843,793, issued July15, 1958. While it is -part of`-,thevil1ustrative.magnetron structure,itis not relatedto the :invention .of the instant application, whichbriefly stated, `relates tor-avarying-pitch constant phase velocity`helical type .of interactioncircuit for a traveling Wave tube.

It is to be understood .that the several arrangements which havebeentdescrihed .are merely illustrativeofthe invention, and that other`modifications may be devised by one skilled in fthe artWithoutxdeparting `from theA spirit and scope of the application. Inparticular, the I.principles described may be extended to 'travelingwave tubesr which employ other .forms of slow wave circuits which likethe helix do .not involve spatial harmonic operation and so do not needto beperiodic in structure, but which ordinarily are made ,periodic instructure to facilitate achieving a Vconstant, phase velocity.

What is claimed is:

l. In a traveling wave tube utilizing a wave transmission circuit for`propagating electromagnetic Waves in field coupling relation with anelectron stream, the combination including a transmission circuitcomprising a helix portion having predetermined variations in pitchalong its length in .the region of said iield coupling relation, andmeans coupled tosaid helix portion for counteractingy the effect of thevariations in `pitch upon the uniformity of phase velocity of a Wavepropagated therealong.

2. A device .according `to claim 1 in which the variations in pitch .ofthe said helix portion are substantially uniform with length therealong.

3. A device according to claim 1 in which the pitch of the helix portionincreases in a single direction therealong.

4. A device Aaccording to claim 1 in which the pitch of the helixportion increases in one direction therealong in one `part of the lengthand in the other direction therealong in another part of the length.

5. A device according to claim 1 in which the means for counteractingthe effect of variations in pitch of the helix portion upon theuniformity of phase velocity comprises a member of dielectric materialcoupled to the helix portion and :electrically loading it variably`along its length.

6. A device according to .claim 5 in which the variable loading of thedielectric .material along the fhelix portion compensates vfor thevariations in Vwave phase velocity therealong which .are ydue to thevariable pitch, and maintain substantially Aconstant -wave phasevelocity along the length of thehelix portion.

7. A device :according to claim l in which the means for counteracting`the effect -of variations in pitch of the said helix portion upon -theuniformity of phase velocity comprises a second helix Vportion coupledtothe said helix portion fhaving predetermined variations in pitch alongits length.

8. A device according to claim 7 in which variations inthe couplingbetween the two helix portions along their lengths compensate for thevariations in wave phase velocity along the lengthof the first saidhelix portion which are due to variable pitch, and maintains constantwave phase velocity along the length of the first saidfhelix portion.

9. A device according to-clairn 7 in which said second helix portion has`predetermined variations in pitch along its length.

10. A device according to claim 7 in which said second helix portion haspredetermined variations in diameter along its length.

l1. A device according to claim 7 in which said second helix portion haspredetermined variations in .pitch and diameter along its length.

12. In a traveling Wave tube utilizing a Wave transmission circuit forpropagating electromagnetic -waves in eld coupling relation with anelectron stream, the combination including a transmission circuitcomprising a helix portion having predetermined variations in pitchalong `its length Vin the region of said ield coupling relation, andmeans coupled to said helix portion for compensating for the variationsin ywave phase velocity therealong which result from the variations inpitch, said means comprising amember of dielectric material dimensionedand coupled in such a way to the helix portion that the wave `phasevelocity remains substantially constant, `therealong.

13. In a traveling wave tube utilizing a wave transmission circuit .forpropagating electromagnetic waves in field coupling relation with anelectron stream, the combination including a transmission circuitcomprising a irst helix having a first portion of uniform pitch, asecond intermediate portion of varying pitch, and a third portion ofuniform pitch, and means coupled to said second intermediate helixportion for compensating .for the variations in wave phase velocitytherealong which result from the variable pitch, said means including asecond helix coupled to said rst helix with a rst portion of uniformpitch, a second intermediate portion of varying pitch, .and a thirdportion of uniform pitch, each `of said portions of said second helixbeing in coupling relation with the respective portions of said firsthelix.

14. A device according to claim 13 in which the variations in `pitch ofthe intermediate portions of said rst and second `helices aresubstantially uniform with length therealong.

15. A device according to claim 13 in which the pitch of theintermediate portions of said lirst and second helices increases in onedirection therealong in one part of the length and in the oppositedirection thcrealong in another part of the length.

16. A device according to claim 13 in which the variations in pitchalong the intermediate portion of said first helix is substantiallyuniform with length therealong and the intermediate portion of saidsecond helix has predetermined variations in pitch and diametertherealong.

References Cited in the le of this patent UNITED STATES PATENTS2,489,082 De Forest Nov. 22, 1949 2,541,843 Tiley Feb. 13, 19512,630,544 Tiley Mar. 3, 1953 2,641,731 Lines June 9, 1953 2,727,179Lally et al. Dec. 13, 1955 2,730,648 Lerbs Jan. 10, 1956 2,773,213 DoddsDec. 4, 1956 2,784,339 Lindenblad Mar. 5, 1957 2,807,744 Lerbs Sept. 24,1957 2,825,841 Convert Mar. 4, 1958 2,851,630 Birdsall Sept. 9, 19582,888,596 Rudenberg May 26, 1959

